One idea could be an OC with the Rinnegan being a transfer student to the Highschool DxD world. Their Rinnegan abilities could attract the attention of different factions. For example, they might be able to use the Preta Path to absorb the demonic energy attacks of the devils, which would both intrigue and alarm the devils in the school.
A plot could revolve around the OC's Rinnegan being the key to an ancient prophecy in the Highschool DxD universe. This could lead to the OC being hunted by some forces who want to prevent the prophecy from coming true, while being protected by others who believe it will bring about a new era of peace or power. Maybe the OC has to learn to control the Rinnegan's powers quickly to survive.
Perhaps the OC with the Rinnegan discovers a hidden connection between the world of Highschool DxD and the Naruto universe from which the Rinnegan originates. This could involve finding a way to use the Rinnegan to travel between the two worlds or unlock secrets that were long forgotten. As a result, the OC becomes a sort of bridge between the two very different universes, causing all kinds of chaos and new alliances to form.
An OC (Original Character) in a Highschool DxD fanfiction having the Rinnegan could be an extremely powerful and unique addition. The Rinnegan, from the Naruto universe, brings a whole set of abilities like the Six Paths powers. In the context of Highschool DxD, this OC could use these powers in various ways. For example, the ability to see chakra could be translated into seeing the magical energy in the DxD world. They could use the gravitational powers of the Rinnegan to manipulate the environment during battles against devils, angels, or fallen angels. It would make for a very interesting and overpowered character in the Highschool DxD fanfiction setting.
One idea could be an OC gunslinger who is actually a reincarnated warrior from a different dimension. They could have a special gun that has the power to nullify the abilities of devils, angels, and fallen angels. This would create a unique dynamic as they try to find their place in the Highschool DxD world while being hunted by different factions who either want to use their power or eliminate them.
A plot could revolve around the Sharingan OC being a target because their Sharingan is seen as a powerful tool. Devils might want to use it for their own gain, and angels might see it as a threat. The OC then has to fight to protect themselves and their friends. Also, the OC could use their Sharingan to unlock a hidden power within Issei, which changes the power balance in the DxD world. Another idea is that the OC comes from a parallel world where the Sharingan and DxD powers are combined, and they are on a mission to bring balance to the DxD world.
A plot could involve the mute OC being a new transfer student in the school. Their muteness makes them an object of curiosity and some characters might initially misjudge them.
The OC's Rinnegan in a Highschool DxD fanfiction would be extremely powerful. It could give the character abilities like seeing through illusions, which would be very useful against devils known for their trickery. The power to summon creatures from the Six Paths could also be a game - changer. It could summon powerful beings to fight alongside the OC in battles against other supernatural entities in the DxD world.
Another plot could involve the OC being sent by an unknown force to either help or hinder Lucifer. They initially seem like an enemy but as the story progresses, it's revealed that they are actually on the same side. They could team up to fight against a common threat to the devil world, like an ancient evil that is starting to rise again.
One idea could be an OC demigod who has the power of a forgotten god. They could use this power to help the main characters in unexpected ways, like having the ability to manipulate time on a small scale during battles. Another plot could involve the OC demigod trying to find their place in the world of Highschool DxD, being half - god and half - something else. They might struggle with acceptance from both the devil and angel factions.
One idea could be an OC who is a gamer and discovers a hidden power within the Highschool DxD universe while playing a virtual reality game related to it. This power could be something unique like the ability to control shadows in the DxD world.
One idea could be an OC who is a hybrid of a devil and a fallen angel. The story could revolve around their struggle to find a place in both worlds. They might face discrimination from pure - blooded devils and fallen angels alike, but also discover unique powers that come with their hybrid nature.
One interesting plot idea could be an OC with a Sharingan who transfers to the Highschool Dxd world. They could use their Sharingan abilities to detect the devils' true forms and powers. For example, during a Rating Game, the OC could analyze the opponents' strategies in an instant using the Sharingan's precognitive abilities.